Active Pixel Sensor ("APS") technology represents a second generation image sensor technology. An Active Pixel Sensor includes image acquiring structure, including an amplifier, in each pixel.
Each pixel has a limited light gathering capability that is proportional to the size of the light gathering structure. However, this image acquiring structure has previously taken up some of the otherwise availible area, or "real estate" on the image sensor substrate, "the chip". The so-called "fill factor" is a measure of how much of the chip is used to gather incoming light. Fill factor is degraded by this associated image acquiring structure. The fill factor of active pixel sensors has been as low as 20 to 30 percent.
One possible solution to this problem is the use of microlenses, such as described in co-pending patent applications, e.g. Ser. No. 08/558,521. Microlenses have been used in an attempt to refract some of the light impinging on the active pixel sensor to a photoreactive location. For example, if the light impinges on an area of the real estate that holds the amplifier, that light may be refracted to the area of the photosensitive element.
However, the inventors noticed problems with using the microlenses.
The shape of the microlens is very difficult to control. Rays having large incident angles may be shifted to the neighboring pixels by the microlenses. The inventors hence found that introducing the microlenses may increase the crosstalk and noise level.
FIG. 1 illustrates this occurrence. A ray 100 off the optical axis of photogate A is shown being reflected at the air/microlens interface. A portion 102 of that incoming ray enters the neighboring microlens B, thereby increasing crosstalk and noise level.
For example, a ray at 60 degrees off the normal incidence may have over 10% reflected at the air/microlens interface to generate the crosstalk. Moreover, about 20% of the s-polarized light will be reflected and create crosstalk, while only less than 1% of p-polarized light is reflected. This may be unacceptable for certain applications.
The inventors recognized the need for a different technique of channeling light to a desired area on a light sensitive device. The preferred technique of the present invention uses light channelling without lensing effects to avoid this problem.
The preferred system uses small sized wedge-shaped reflectors ("micro wedge reflectors") to accomplish this goal. These reflectors can increase the photon collection efficiency without generating crosstalk.
The micro-wedge reflectors preferably cover the area of the associated image structure of the APS. This channels many of the image photons into the photosensitive areas, to thereby enhance the photon collection efficiency.
Another aspect of the invention limits the photons to movement in one direction. This further decreases the crosstalk.
For small pixel sizes close to the diffraction limit, for example, microlenses may not work. All lenses have inherent properties. For example, a lens cannot focus more distinctly than its diffraction limit. A lens also cannot focus to achieve resolution beyond what the respective space-bandwidth product will allow. The quality of a lens is usually inversely proportional to its cost. This has provided a lower limit on the size of pixels that can be used with microlenses.
The wedge reflectors of the present invention are reflective, not focusing, structures. Hence, there is no limits on size or resolution.
The micro-wedge structure described herein is based on the inventor's recognition of the crosstalk and other drawbacks with microlenses. This structure uses the reflection effect to avoid the undesired refraction effect. This structure may allow better efficiency than the microlens. It can be precisely fabricated by micro-fabrication technology, it can work well for very small pixels, and there is smaller ray shifting for large incident angle rays. Since substantially all of the photons are limited to redirection in one direction, the crosstalk can be minimized.